The present invention relates to a method, apparatus and program for alarming decrease in tire air-pressure.
It is utilized in a deflation warning system, DWS that a dynamic load radius is lessened for decreased tires and rotational wheel speed of tires with decreased pressure is faster than that of tires with normal inner pressure.
In the DWS, a deflation judgment value, DEL is calculated from the rotational wheel speeds of 4 wheel tires, for example, by the following equation (1);DEL={(V1+V4)/2−(V2+V3)/2}÷{(V1+V2+V3+V4)/4}×100(%)  (1)(In the equation (1), V1 to V4 show the rotational wheel speeds V1 of a front left tire, V2 of a front right tire, V3 of a rear left tire and V4 of a rear right tire, respectively.). It is judged whether the tire air-pressure is decreased or not, and when it is judged that the air-pressure of even one wheel is decreased, an alarm is generated.
Hereat, when all of 4 wheel tires are normal inner pressure at straight running, all of V1 to V4 are the same value; therefore DEL calculated by the equation (1) is zero. On the other hand, when a car turns round a corner, centrifugal force is applied, load is not equally applied to all of 4 wheel tires even if all of 4 wheel tires are normal inner pressure and the dynamic load radii of 4 wheel tires come to differ. Accordingly, V1 to V4 are not wholly the same value in the equation (1), and the deflation judgment value DEL is not “0” that is a value at normal inner pressure at straight running and approaches to a threshold A for judging decrease in tire air-pressure in like manner as a case that tire air-pressure is decreased. What is even worse, it exceeds occasionally the threshold A for judging decrease in tire air-pressure. Consequently, detection precision is improved by correcting the deflation judgment value (DEL) (refer to Japanese Unexamined Patent Publication No. 2002-12013) during turning round a corner.
However, the correction is a correction appropriately used when a car turns round a corner without a bank angle. Further, when a car turns round a corner with a bank angle, the load distribution of each of 4 wheels is different from the load distribution at a case that a car turns round a corner without a bank angle; therefore the rotational wheel speed of each of 4 wheels in case that a car turns round a corner with a bank angle is different from the rotational wheel speed of each of 4 wheels in case that a car turns round a corner without a bank angle. Accordingly, if correction similar as correction carried out when a car turns round a corner without a bank angle is also carried out when a car turns round a corner with a bank angle, erroneous report occurs occasionally. When the bank angle is small, difference between a judgment at this case and a judgment value “0” at normal inner pressure is small even if the correction is inappropriate; therefore erroneous report is not made, but there has been a problem that when the bank angle is certain angle or more, the judgment value exceeds an alarming threshold to make erroneous report.